In recent years, new environmental regulations have forced engine manufacturers to modify engine compression and timing to control soot particulate and oxides of nitrogen (NOx) emissions better. Engine manufacturers have modified their designs to improve emissions controls. Further regulations have triggered wide-spread use of exhaust emission catalyst systems, some to trap carbonaceous particulates and others to decompose NOx and hydrocarbons. The combination of engine operating conditions and emission control system changes, particularly the introduction of exhaust gas recirculation (EGR), has increased the soot loading of diesel engine lubricants. The soot particulates can be quite hard and abrasive. Consequently, the increased soot loading in modern diesel engines has contributed to higher valve train wear rates.
Since some engine manufacturers believe that phosphorus and sulfur may poison exhaust catalysts, a simultaneous trend has been the reduction of phosphorus and sulfur content of heavy duty motor oils (HDMO). Phosphorus and sulfur may poison emission control catalysts. Phosphorus comes from zinc alkyl dithiophosphate (ZnDTP), the predominant antiwear agent for the past 50 years. Sulfur, too, is present in ZnDTP and in most commercial detergents, either sulfonates or phenates. The detergents are key components of engine oils, present to neutralize acidic oxidation products and to suspend varnish and sludge deposits. In order to protect and extend the life of the emission control catalysts, engine manufacturers are requesting lubricants with ever lower phosphorus and sulfur levels. Therefore, ZnDTP and traditional sulfur-based detergents are being replaced with additives that do not contribute phosphorus or sulfur.
Reduction of the phosphorus and sulfur content while increasing the soot-loading of diesel engine oils in the field present fundamental changes in formulating strategy. A further complication is that fully-formulated lubricants are time-consuming and costly to develop. A typical engine test program for the American Petroleum Institute (API) Cl-4 specification can easily exceed $1,000,000.
Bench testing can be a cost-effective alternative to full scale engine tests. Bench tests can moderate costs by simulating valve train wear in small, inexpensive rigs. Thus, a bench test is generally inexpensive, perhaps only $100. To be of use to the formulator or additive synthesis chemist and to reduce engine test costs, the bench test must be related to a key engine test parameter. This is most often done by the careful selection of reference oils with known performance in that key engine test. In the case of a heavy duty valve train wear test, the proper selection of appropriate bench test reference fluids and surrogate soot are vital.
Ideally, a diesel wear bench test would use actual engine soot. However, engine soot is difficult to isolate from used engine oil, requiring dilution and high speed centrifugation to precipitate the soot, then further washing steps to remove residual oil and additives. The amount of isolated soot is low, less than 10% by weight of the original oil sample. The precipitated soot will contain additives or additive fragments that washing will not remove. Overall, engine soot is time-consuming to isolate and prepare, highly variable in composition (because of the engine oil additives), and available in limited quantity.
Carbon black is an attractive, practical alternative to engine soot. Carbon black is globally available in commercial quantities. Its chemical and physical properties can be selected to mimic those of the engine soot of interest. Overall, carbon black is a cost- and time-effective alternative to engine soot for bench testing applications. Thus, it is highly desirable to establish a bench test using carbon black that can reliability predict soot performance of lubricating oils.